Wednesday 17 December 2014

Antenna & Wave Propagation

Antenna & Wave propagation  are related with each other. They are discussed separately first as Antenna and second as Wave Propagation.
1.Antenna:- An antenna is an electrical device which converts electric power into radio waves, and vice versa.It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. a high frequency alternating current (AC)) to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. E.g. radio broadcasting,broadcast television,satellite communications, radar, cell phones, Bluetooth-enabled devices.

The origin of the word antenna relative to wireless apparatus is attributed to Italian radio pioneer Guglielmo Marconi . In the summer of 1895, Marconi began testing on his wireless system outdoors on his father's estate near Bologna and soon began to experiment with long wire "aerials". Marconi discovered that by arranging these "aerials" vertically and placing them in the earth (grounding them) that the range of his wireless system was significantly increased.Soon he was able to transmit signals over a hill, a distance of approximately 2.4 kilometers . In Italian a tent pole is known as l'antenna central, and the pole with the wire was simply called l'antenna.

According to their applications and technology available, antennas generally divide in one of two categories :-
Yagi-Uda Antenna

A. Omnidirectional :-Omnidairection directional antennas which receive or radiate more or less in all directions. These are employed when the relative position of the other station is unknown or arbitrary. They are also used at lower frequencies where a directional antenna would be too large. E.g. Vertical antenna or whip antenna.

B. Directional :- Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern. E.g. Dipole Antenna.

2.Wave propagation:- Wave propagation is any of the ways in which waves travel.

With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves.
EM Field Of A Dipole Antenna
For electromagnetic waves, propagation may occur in a vacuum as well as in a material medium. Other wave types cannot propagate through a vacuum and need a transmission medium to exist.

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